In the early 1980’s, an awareness of the
benefits of thrust vectoring for dramatically improved control at high
angles of attack surfaced. In addition to studies of advanced engine
concepts with vectoring nozzles, interest arose over the use of simple
thrust-vectoring paddles in the engine exhaust to deflect the thrust for
control augmentation. As discussed in Langley Contributions to the F-14,
the Navy, with Langley’s assistance, had taken the lead in this area with
flight tests on an F-14 modified with single-axis yaw-vectoring paddles.
In addition, during a cooperative program with Rockwell led by Langley
researcher Bobby L. Berrier, Langley provided design data for multi-axis
thrust-vectoring paddle configurations using the Jet Exit Test Facility in
1985. Based on these fundamental research studies, Rockwell incorporated
multi-axis thrust-vectoring paddles into the SNAKE configuration.
Free-flight tests of the modified SNAKE model in the Full-Scale Tunnel by
Croom’s team in 1985 provided an impressive display of the effectiveness
of thrust vectoring at extreme angles of attack.

In West Germany, Dr.
Wolfgang Herbst of Messerschmitt-Bolkow-Blohm (MBB) aggressively touted
the advantages of post-stall technology (PST) for increased effectiveness
during close-in air combat. Herbst’s conclusions were based on wind-tunnel
tests of a German advanced canard fighter configuration known as the
TKF-90 and piloted simulator studies during which the application of
simulated thrust vectoring resulted in rapid directional turns at high
angles of attack had increased the turn rate by over 30 percent. Technical
discussions between the Rockwell SNAKE Program managers and Herbst were
initiated in 1983, and planning for a mutual program on PST ensued.
Discussions with the Defense Advanced Research Projects Agency (DARPA)
were very positive. When funding for collaborative international
activities became available from the U.S. (the Nunn-Quayle research and
development initiative in 1986) and West German governments, the technical
expertise of Rockwell and MBB were joined under DARPA sponsorship in the
X-31 Program. In view of Langley’s extensive experience in
high-angle-of-attack technology, unique test facilities, and contributions
to the Rockwell SNAKE Program, DARPA requested in 1986 that Langley become
a participant in the X-31 development program.

Langley Flight Test Center
NASA

X-31

Type

Experimental

Manufacturer

Rockwell
Messerschmitt-Bölkow-Blohm

Maiden flight

1990

Primary users

DARPA
NASA

Number built

2

Click on Picture to enlarge

The X-31 aircraft returns from a test flight for
VECTOR.

A diagram of the Herbst maneuver. (NASA)

The X-31 showing its three
thrust vectoring paddles.

The X-31 in Oberschleißheim

The three thrust vectoring paddles.

The collaborative
U.S.-German
Rockwell-MBB
X-31 Enhanced Fighter Maneuverability program was designed to
test fighter thrust vectoring technology. Thrust vectoring allows the X-31
to fly in a direction other than where the nose is pointing, resulting in
significantly more maneuverability than most conventional fighters. An
advanced flight control system provides controlled flight at high
angles of attack where conventional aircraft would
stall.

X-31 History

Two X-31s were built, and over 500 test flights were carried out between
1990 and 1995. The X-31 featured fixed strakes
along the aft
fuselage, as well as a pair of movable computer-controlled
canards to increase stability and maneuverability. There are no
horizontal tail surfaces, only the vertical fin with
rudder.
Pitch and yaw are controlled by the three paddles directing the exhaust
(thrust vectoring). Eventually, simulation tests on one of the X-31s showed
that flight would have been stable had the plane been designed without the
vertical fin, because the thrust-vectoring nozzle provided sufficient yaw
and pitch control.

During flight testing, the X-31 aircraft established several milestones.
On
November 6,
1992, the X-31 achieved controlled flight at a 70-degree angle of
attack. On
April 29, 1993,
the second X-31 successfully executed a rapid minimum-radius, 180-degree
turn using a
post-stall maneuver, flying well beyond the aerodynamic limits of any
conventional aircraft. This revolutionary maneuver has been called the "Herbst
maneuver" after
Dr. Wolfgang Herbst, an MBB employee and proponent of using post-stall
flight in air-to-air combat.[1]
Herbst was the designer of the
Rockwell SNAKE, which formed the basis for the X-31.[2]

In the mid-1990s, the program began to revitalize and a $53 million
VECTOR program was initiated capitalizing on this previous investment.
VECTOR is a joint venture that includes the US Navy, Germany’s defense
procurement agency BWB, Boeing's
Phantom Works, and the European
Aeronautic, Defense and Space Company in Ottobrunn, Germany. As the site
for the flight testing,
Naval Air Station Patuxent River in Maryland was chosen. From 2002 to
2003, the X-31 flew extremely short takeoff and landing approaches first on
a virtual runway at 5,000 feet in the sky, to ensure that the
Inertial Navigation System/Global
Positioning System accurately guides the aircraft with the centimeter
accuracy required for on the ground landings. The program then culminated in
the first ever autonomous landing of a manned aircraft with high angle of
attack (24 degree) and short landing. The technologies involved a
differential GPS System based on pseudolite technology from Integrinautics,
California, and a miniaturized flush air data system from Nordmicro,
Germany.

Wikipedia

X-31 Enhanced Fighter Maneuverability
Demonstrater

Two X-31 Enhanced Fighter Maneuverability (EFM) demonstrators flew at the
NASA Dryden Flight Research Center, Edwards, CA, and at Palmdale, CA, to
obtain data that may apply to the design of highly maneuverable
next-generation fighters. The program ended in June 1995.

Click on Picture to enlarge

The X-31 program demonstrated the value of thrust vectoring (directing
engine exhaust flow) coupled with advanced flight control systems, to provide
controlled flight during close-in air combat at very high angles of attack.
The result of this increased maneuverability was a significant advantage over
most conventional fighters.

Background

"Angle of attack" (AoA or alpha) is an engineering term used to describe
the angle of an aircraft's wings relative to the incoming wind direction.
During maneuvers, pilots often fly at extreme angles of attack — with the nose
pitched up while the aircraft continues in its original direction. This can
lead to loss of control and result in the loss of the aircraft, pilot, or
both.

Three thrust vectoring paddles mounted on the X-31's airframe adjacent to
the engine nozzle directed the exhaust flow to provide control in pitch
(moving the nose up or down) and yaw (moving it right or left) to improve
control. Made of carbon-carbon -- an advanced carbon fiber reinforced
composite -- the paddles could sustain temperatures of up to 1,500 degrees
celsius for extended periods. In addition the X-31s were configured with
movable forward canards and eventually with fixed aft strakes. The canards
were small wing-like structures set on a line between the nose and the leading
edge of the wing. The strakes were set on the same line between the trailing
edge of the wing and the engine exhaust. Both supplied additional pitch
control in tight maneuvering situations.

Click on Picture to enlarge

The X-31 operated with a digital fly-by-wire flight control system. It
included four digital flight control computers with no analog or mechanical
back-up. Three synchronous main computers drove the flight control surfaces.
The fourth computer served as a tie-breaker in case the three main computers
produced conflicting commands.

The X-31 research program produced technical data at high angles of attack.
This information gave engineers and aircraft designers a better understanding
of aerodynamics, effectiveness of flight controls and thrust vectoring, and
airflow phenomena at high angles of attack. This may lead to design methods
providing better maneuverability in future high-performance aircraft and make
them safer to fly.

The X-31 program was divided into three phases:

Phase 1

Phase 1 was the conceptual design phase. During this phase, program
personnel outlined the payoff expected from the application of EFM concepts in
future air battles and defined the technical requirements for a demonstrator
aircraft.

Phase 2

Phase 2 carried out the preliminary design of the demonstrator and defined
the manufacturing approach. Three governmental design reviews took place
during this phase to examine the proposed design. Technical experts from the
U.S. Navy, German Federal Ministry of Defense, and NASA contributed to the
careful examination of all aspects of the design.

Phase 3

Phase 3 initiated and completed the detailed design and fabrication of two
aircraft, which were assembled at the Rockwell International (now Boeing)
facility at Air Force Plant 42, Palmdale, CA. This phase required that both
aircraft fly a limited flight test program. The first aircraft was rolled out
on Mar. 1, 1990, followed by a first flight on Oct. 11, 1990, piloted by
Rockwell chief test pilot Ken Dyson. The aircraft reached a speed of 340 mph
and an altitude of 10,000 feet during its initial 38-minute flight.

The second aircraft made its first flight on Jan. 19, 1991, with Deutsche
Aerospace chief test pilot Dietrich Seeck at the controls. Despite the fact
that the number one and two aircraft had identical external dimensions, X-31
number two experienced stronger yaw asymmetries than aircraft number one. For
this reason, X-31 team members began to refer to aircraft number two as the
"evil twin." The team tested aircraft number two with varying lengths of
extended nose strakes and found that it could get the two aircraft to fly
identically with 8 1/2 inches of strake length on the second X-31, making it
an evil twin no longer.

X-31 Flight Summary

During the program's initial phase of operations at Rockwell
International's Palmdale facility, pilots flew the aircraft on 108 test
missions. They achieved thrust vectoring in flight and expanded the post-stall
envelope to 40 degrees angle of attack before flight operations were moved to
Dryden in February 1992 at the request of the Defense Advanced Research
Projects Agency (DARPA). (Stall is a condition of an airplane or an airfoil in
which lift decreases and drag increases due to separation of airflow. Thrust
vectoring compensated for the loss of control through normal aerodynamic
surfaces that occurred during a stall. Post-stall refers to flying beyond the
normal stall angle of attack, which in the X-31 was at 30 degrees angle of
attack.)

Because of the basic stability of the aircraft at high angle of attack, it
exhibited low tolerance for sideslip. This became a problem at higher angles
of attack. Below 30 degrees AoA, the nose boom updated the inertial navigation
unit with air data. When the pilots began flying for extended periods above 30
degrees AoA, the inertial navigation unit began calculating large but
fictitious values of sideslip as a result of changes in wind direction and
magnitude.

Click on Picture to enlarge

To solve this problem, the X-31 team incorporated an unusual noseboom
called a Kiel probe instead of the standard NASA pitot tube to calculate air
flow. The Kiel probe was bent 10 degrees downward from the standard pitot
configuration. The team also had to rotate the sideslip vane down 20 degrees
from the noseboom to counter an oscillation that occurred at 62 degrees AoA.
With these modifications, the air data to the inertial navigation unit was
accurate throughout the AoA envelope, eliminating the problem of false
readings of sideslip drift at high AoA.

At Dryden an international team of pilots and engineers (ITO: International
Test Organization) expanded the aircraft's flight envelope, including military
utility evaluations that pitted the X-31 against comparable but non-thrust
vectored aircraft to evaluate the maneuverability of the X-31 in simulated air
combat. The ITO included participation by NASA, the U.S. Navy, the U.S. Air
Force, Rockwell Aerospace, the Federal Republic of Germany, and Daimler-Benz
(formerly Messerschmitt-Bolkow-Blohm and Deutsche Aerospace).

The first flight from Dryden under the ITO was in April 1992, and by July
1992 the X-31 program was continuing the initial stage of post-stall envelope
expansion.

Throughout the process of envelope expansion, the team learned more about
the aircraft and had to make many modifications to the control laws (aircraft
equations of motion that governed the control of the airplanes through the
flight control computers). This was necessary because the actual aerodynamics
of the aircraft were slightly different from what the wind tunnels had
predicted. The X-31 team was able to make these changes quickly, so they
rarely delayed the process of envelope expansion.

When the pilots started flying above 50 degrees AoA, they encountered kicks
from the side that they called lurches. The international team added narrow
1/4-inch-wide strips of grit to the aircraft's noseboom and radome to change
the vortices flowing from them. The grit strips reduced the randomness of the
lurches caused by the vortices, enabling the pilots to finish envelope
expansion to the design AoA-limit of 70 degrees at 1 g of acceleration.

As the pilots began flying entries to post-stall angles of attack, they
experienced unintentional departures from controlled flight at 58 degrees AoA
during maneuvers known as a split-s. Analysis by engineers indicated that the
departures were caused by very large asymmetries in yaw (movement of the nose
to the right or left)—so large that they overcame the power of the available
thrust vector controls.

Previous AoA research and new wind-tunnel testing with an X-31 model led
the research team to add strakes to the nose that were 6/10 of an inch wide by
20 inches long. The team also blunted the nose tip slightly (increasing the
radius of curvature from essentially zero to 3/4 of an inch). The strakes
forced more symmetric transition of forebody vortices and the blunted nose tip
reduced yaw asymmetries. Both changes constituted significant improvements to
the aircraft's aerodynamics, but the blunting of the nose tip was the more
significant of the two.

Click on Picture to enlarge

A diagram of the Herbst maneuver. (NASA)

The No. 2 X-31 achieved controlled flight at 70 degrees angle of attack at
Dryden on Nov. 6, 1992. On that same day, it performed a controlled roll
around the aircraft's velocity vector at 70 degrees angle of attack.

On April 29, 1993, the No. 2 X-31 successfully executed a minimum radius,
180-degree turn using a post-stall maneuver, flying well beyond the
aerodynamic limits of any conventional aircraft. The revolutionary maneuver
has been dubbed the "Herbst Maneuver," after Wolfgang Herbst, a German
proponent of using post-stall flight in air-to-air combat. The maneuver has
also been described as a "J" turn when flown to an arbitrary heading change.

During the final phase of evaluation, with the X-31's engaged in simulated
air combat scenarios against adversaries flying F/A-18's and some other
tactical aircraft, the X-31's were able to outperform other aircraft without
thrust vectoring through use of post-stall maneuvers.

The X-31 constituted a revolution in air combat in the post-stall region.
The pilots in the program did not support trading off other important fighter
characteristics just to acquire the EFM capabilities the X-31 possessed. But
they did conclude that the improved pitch pointing and velocity-vector
maneuvering permitted by thrust vector control did provide new options for the
pilot to use in close-in combat. Post-stall maneuvering allowed the pilots to
rotate and point the nose of the vehicle at the adversary aircraft in such a
way that the adversary pilot could not counter the maneuver. But this was true
only when used selectively and rapidly. The X-31 also greatly improved flight
safety since it was fully controllable and flyable in the post-stall region,
unlike other fighter aircraft without thrust vectoring.

Click on Picture to enlarge

Despite this greater safety, the No. 1 X-31 aircraft was lost in an
accident Jan. 19, 1995. The pilot, Karl Heinz-Lang, of the Federal Republic of
Germany, ejected safely before the aircraft crashed in an unpopulated desert
area just north of Edwards. The crash resulted from an unexpected single-point
failure in the noseboom airspeed system.

The X-31 program logged an X-plane record of 580 flights during the
program, 559 research missions and 21 in Europe for the 1995 Paris Air Show. A
total of 14 pilots representing all agencies of the ITO flew the aircraft.

Quasi-Tailless Demonstration

In 1994, software was installed in the X-31 to simulate the feasibility of
stabilizing a tailless aircraft at supersonic speed using thrust vectoring.
Tests also included subsonic speeds. During the flights the pilot destabilized
the aircraft with the rudder to stability levels that would be encountered if
the aircraft had a reduced-size vertical tail. The X-31 quasi-tailless flight
test experiment demonstrated the feasibility of tailless and reduced-tail
fighters. Had the capability been part of the airplane's design from the
beginning, the benefits had the potential to outweigh the complexity the
concept entailed. More
Information down the page

The 1995 Paris Air Show

The X-31's enhanced maneuvering capabilities were demonstrated to the
international aerospace industry during daily flights at the 1995 Paris Air
Show. These flights featured post-stall maneuvers at low altitudes. The
aircraft flew to Europe aboard a U.S. military C-5 transport. A small team of
NASA and industry personnel supported it there.
More Information down the page

Program Management

An international test organization of about 110 people, managed by the
Defense Advanced Research Projects Agency (DARPA—re-designated ARPA from March
1993 to 1996), conducted the flight tests at Dryden and Palmdale. NASA was
responsible for flight test operations, aircraft maintenance, and research
engineering after the project moved to Dryden. As the research flight program
matured, the test organization declined in size to approximately 60 persons.

The X-31 was the first international experimental aircraft development
program administered by a U.S government agency and was a key effort of the
NATO Cooperative Research and Development Program.

The ITO director and NASA's X-31 project manager at Dryden was Gary
Trippensee.

Aircraft Specifications

Designed and constructed as a demonstrator aircraft by Rockwell
International Corporation's North American Aircraft and Deutsche Aerospace,
the X-31 had a wing span of 23.83 feet. The fuselage length was 43.33 feet.

The X-31 was powered by a single General Electric F404-GE-400 turbofan
engine, producing 16,000 pounds of thrust in afterburner.

Typical takeoff weight of the X-31 was 16,100 pounds including 4,100 pounds
of fuel.

The X-31 design speed was Mach 0.9 with an altitude capability of 40,000
feet. For specific tests to determine thrust vectoring effectiveness at
supersonic speeds the aircraft was flown to Mach 1.28 at an altitude of 35,000
feet.

A common requirement for fighter planes is the ability to reach high roll
angle accelerations, as this parameter is one of most important determinants
of the plane's maneuverability.

Fig.1 - Pressure coefficient distribution for a sample wing design

If a plane has two edge flaps on the wings, deflecting the two
independently will cause a rolling moment to ensue, which in turn induces a
roll acceleration. Of course, key factors in determining the roll acceleration
are the authorities of the inboard and outboard flaps, and the position of the
flap split.

Unfortunately, these parameters cannot be chosen at will, because during
the maneuver aerodynamic and mass loads are imposed on the structure,
creating stress that the structure must be able to withstand. To guarantee the
structural integrity is then necessary to add material, increasing the wing
weight and reducing the range.

Accordingly, the solution must be a good compromise between good roll
performance and low structural mass.

Mode Frontier's Contribution To Its Solution

To address this problem, DASA used modeFRONTIER to optimise the flap
splitting (one discrete variable) and the flap settings (two continuous
variables) on the X31 wing model.

Click on Picture to enlarge

Fig.2 - The X31 experimental aircraft

Two conflicting objectives were pursued:

maximum roll angle acceleration

minimun wing mass

DASA's HISSS-D subsonic and supersonic solver was used for the aeroelastic
design evaluation, while the structural weight for a given configuration was
minimized, while maintaining structural integrity, using LAGRANGE, another
proprietary code. The software run on an SGI Origin2000 parallel computer.

Click on Picture to enlarge

Fig.3 - Objective space for the designs found by mode FRONTIER.

Optimization results using 16 individuals and 8 generations - using the
MOGA algorithm - were sufficient to precisely describe the Pareto Frontier,
thus singling out the set of optimal designs for different trade-offs.

For a given weight it became then possible to pick the design providing the
highest acceleration possible. Conversely, given a desired roll acceleration
it was possible to immediately find the design with the lowest weight.

The X-31 Enhanced Fighter Maneuverability (EFM) demonstrator, flown at
NASA's Dryden Flight Research Center, Edwards, Calif., provided information
which is invaluable for proceeding with the designs of the next generation
highly maneuverable fighters. The X-31 program showed the value of using
thrust vectoring (directing engine exhaust flow) coupled with advanced flight
control systems, to provide controlled flight to very high angles of attack.
The result is a significant advantage over conventional fighters in a
close-in-combat situation.

Click on Picture to enlarge

An international test organization, managed by the Advanced Research
Projects Agency (ARPA), is conducting the flight tests. In addition to ARPA
and NASA, the International Test Organization (ITO) includes the U.S. Navy,
the U.S. Air Force, Rockwell Aerospace, the
Federal Republic of Germany and Deutsche Aerospace. About 110 people from
the ITO agencies are assigned to the program. NASA is responsible for flight
test operations, and aircraft maintenance. Research engineering is an ITO team
effort. The X-31 is the first international experimental aircraft development
program administered by a US government agency. It is one, if not the most,
successful effort initiated by the NATO Cooperative Research and Development
Program.

"Angle-of-attack" (alpha) is an engineering term to describe the angle of
an aircraft's body and wings relative to its actual flight path. During
maneuvers, pilots would like to fly at extreme angles of attack to facilitate
rapid turning and pointing against an adversary. With older aircraft designs,
entering this flight regime often led to loss of control, resulting in loss of
the aircraft, pilot or both.

Three thrust vectoring paddles made of graphite epoxy and mounted on the
X-31's aft fuselage are directed into the engine exhaust plume to provide
control in pitch (up and down) and yaw (right and left) to improve
maneuverability. The paddles can sustain temperatures of up to 1,500 degrees
centigrade for extended periods of time. In addition, the X-31s is configured
with movable forward canards, wing control surfaces, and fixed aft strakes.
The canards are small wing-like structures located just aft of the nose, set
on a line parallel to the wing between the nose and the leading edge of the
wing. Normally "weathervaned" with the prevailing airflow, these devices are
programmed to be used for aerodynamic recovery from high angles of attack in
event of thrust vectoring system failure. The strakes are set along the same line between the
trailing edge of the wing and the engine exhaust. The strakes supply
additional nose down pitch control authority from very high angles of attack.
Small fixed nose strakes are also employed to help control sideslip.

The X-31 flight demonstration program was focused on agile flight within
the post-stall regime, producing technical
data to give aircraft designers a better understanding of aerodynamics,
effectiveness of flight controls and thrust vectoring, and airflow phenomena
at high angles of attack. This is expected to lead to design methods providing
better maneuverability in future high performance aircraft and make them safer
to fly.

Phase One

Phase I was the conceptual design phase. During this phase the payoff
expected from the application of EFM concepts in future air battles was
outlined and the technical requirements for a demonstrator aircraft were
defined.

Phase Two

Phase II carried out the preliminary design of the demonstrator and defined
the manufacturing approach to be taken. Three major
government design reviews were held during the phase to thoroughly examine
the proposed design. Technical experts from the U.S. Navy, Federal Ministry of
Defense and NASA all contributed to the careful examination of all aspects of
the design.

Phase Three

Phase III initiated and completed the detailed design fabrication and
assembly of two aircraft. This phase required that both aircraft fly a limited
test flight program. The first aircraft rolled out on March 1, 1990, followed
by a first flight at Air Force Plant 42, Palmdale, Calif., on Oct. 11, 1990.
The aircraft was piloted by Rockwell chief test pilot Ken Dyson, and reached a
speed of 340 mph and an altitude of 1 0,000 feet during the initial 38-minute flight.
The second aircraft made its first flight on Jan.19, 1991, with Deutsche
Aerospace chief test pilot Dietrich Seeck at the controls.

Flight Summary

During the program's initial phase of flight test operations at the
Rockwell Aerospace facility in Palmdale, Calif., the two aircraft were flown
on 108 test missions, achieving thrust vectoring in flight and expanding the
post-stall envelope to 40 degrees angle of attack. Operations were then moved
to Dryden in February 1992 at the request of the Advanced Research Projects
Agency (ARPA). At Dryden, the International Test Organization (ITO) expanded
the aircraft's flight envelope, including military utility evaluations that
pitted the X-31 against similarly equipped aircraft to evaluate the
maneuverability of the X-31 in simulated combat. The ITO, managed by the
Advanced Research Projects Agency (ARPA), includes NASA, U.S. Navy, the U.S.
Air Force, Rockwell Aerospace, the Federal Republic of Germany, and Deutsche
Aerospace (formerly Messerschmitt-Bolkow-Blohm).

The first NASA flight under the ITO took place in April 1992. By July 1992,
the X-31 program was continuing the initial stage of post stall envelope
expansion. The X-31 achieved controlled flight at 70 degrees angle of attack
at Dryden on Nov. 6, 1992. On the same day, a controlled roll around the
aircraft's velocity vector was accomplished at 70 degrees angle of attack. On
April 29, 1993, the No. 2 X-31 successfully executed a rapid minimum radius,
180-degree turn using a post-stall maneuver, flying well beyond the
aerodynamic limits of any conventional aircraft. The revolutionary maneuver
has been dubbed the "Herbst Maneuver," after Wolfgang Herbst, a German
proponent of using post-stall flight in air-to-air combat. The term "J Turn"
is also used to describe this type of maneuver, when flown to an arbitrary
heading change.

The first tactical maneuver with a cooperative F/A-18 as adversary was
accomplished in June 1993. In August 1993, the X-31 demonstrated full
capability in flying Basic Fighter Maneuvers. In October 1993 the program
logged its 300th flight. The final tactical evaluation phase, consisting of
Close-In-Combat (CIC) tests with un-choreographed flights against the F/A-18
adversary, began in November 1993. During November and December 1993 the X-31
also reached supersonic speed (Mach 1.28). A total of 160 flights were
completed by the X-31 program in 1993 setting a new annual experimental
aircraft record. One of the two X-31s flew 103 of those flights. The program
also set a new monthly record of 21 research flights in August 1993.

The evaluation of the X-31's unique capabilities in close combat (CIC) was
completed on March 1, 1994. Evaluation of the X-31 as a fighter
maneuverability demonstrator by the ITO concluded in early 1995.

The No. 1 X-31 ship was lost in an accident Jan. 19, 1995. The pilot, Karl
Lang, ejected safely at 18,000 feet before the aircraft crashed into an
unpopulated region of the desert just north of Edwards Air Force Base. There
was no private property damage.

Quasi-Tailles Flight Demonstration

Click on Picture to enlarge

Quasi-Tailless....

In 1994, software was installed in the X-31 to demonstrate the feasibility
of stabilizing a tailless aircraft at supersonic speed, using thrust
vectoring. This software allows destabilization through the control laws of
the aircraft in incremental steps to the goal of simulation 100 percent
tail-off. Quasi-tailless tests began in 1994. The first phase started with
supersonic evaluations at Mach 1.2. Later subsonic evaluations were performed.
During the flights the aircraft was destabilized with the rudder to stability
levels that would be encountered if the aircraft had a reduced size vertical
tail.

The quasi-tailless testing provided data to industry on the benefits of
drag reduction, radar cross section, and weight reduction that could be used
for future commercial and military designs and modifications. Simulated
tailless X-31 flight tests conducted for the Joint Strike Fighter program
successfully provided an initial demonstration that thrust vectoring could
provide yaw control and, thus, reduce or eliminate the need for an aircraft
vertical tail.

Helmet Mounted Visual / Audio Display

Click on Picture to enlarge

Click on Picture to enlarge

Installation of a Helmet Mounted Visual/Audio Display (HMVAD) was completed
on the X-31 (aircraft No. 2) in October 1993. The purpose of the HMVAD is to
provid

e out-of-the-cockpit situation awareness and a simulated
helmet-mounted sight to the pilot during high angle of attack combat
maneuvering.

The system
consists of a GEC Viper helmet with symbology projected on its visor by a
monocular CRT. Also included is a Polhemus head tracker and an angle-of-attack
audio cueing device. Both of these features have been demonstrated on the
X-31, during post-stall close-in-combat, a first for any aircraft. This
equipment will be the baseline for a follow-on virtual adversary program to
demonstrate the feasibility of combat training against onboard and up-linked
targets displayed by the helmet.

The Vectoring Extremely Short Take-Off and Landing Control Tailless
Operation Research (VECTOR) program is a low-cost, highly leveraged approach
to developing and demonstrating thrust vectoring and supporting technologies
to enable complete flight control/engine/thrust vectoring
integration for ESTOL and tailless flight. The Navy is particularly
interested in thrust vectoring benefits in its unique take-off and landing
environment. Germany is interested in the integrated FCS design and a major
supporting technology, an Advanced Air Data System (AADS), which provides
accurate air data information throughout the AOA range. Current systems
develop inaccuracies at high AOA.

In February 1998, the participating contractors started VECTOR Risk
Reduction and Requirements Definition. Efforts included defining detailed
program and design requirements, identifying technology risks and performance
goals, as well as agreeing on work and cost share in the Technology
Demonstration Program. An enabling milestone was accomplished in April 1999
with the signature of the Memorandum of Agreement (MoA) between the German and
U.S. Governments to conduct the VECTOR program.

In previous testing, the X-31 provided data for air combat maneuvering. In
the VECTOR program, however, the aircraft will be exploring thrust vectoring
technology in the take off and landing environment. In that flight regime,
thrust-vectoring technologies may have a potentially significant pay-off in a
number of critical areas, including operational capability, performance,
safety, vehicle complexity, maintenance, and total cost of ownership.

The Boeing Company is responsible to the Naval Air Systems Command (NAVAIR)
for VECTOR program integration and the flight control system hardware effort.
DaimlerChrysler Aerospace of Germany is responsible to the German Federal
Office for Defense Technology and Procurement (BWB) for the flight control law
software, Advanced Air Data System development, simulation build-up, and
aircraft wings and thrust vectoring vanes. In partnership with Dasa, Boeing
will lead X-31 aircraft re-activation, modification, maintenance, repair, and
flight test technology. Boeing is also responsible for the Extremely Short
Take-Off and Landing activities. This effort includes development and
integration of highly accurate navigation equipment. Reduced Tail studies are
pursued in a joint effort of Boeing and Dasa. Major subcontractors of the
VECTOR program include IntegriNautics, Honeywell, RJK Technologies, Moog, and
Nord-Micro. Government participants include the Naval Air Systems Command, the
German federal test center for military aircraft (WTD 61), and the German
aerospace research center (DLR). Flight testing will be led by NAVAIR at the
Patuxent River facility.

Click on Picture to enlarge

An international test organization, managed by the Advanced
Research Projects Agency (ARPA), is conducting the flight tests. In addition
to ARPA and NASA, the International Test Organization (ITO) includes the U.S.
Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany
and Deutsche Aerospace. About 110 people from the ITO agencies are assigned to
the program. NASA is responsible for flight test operations, and aircraft
maintenance. Research engineering is an ITO team effort.
The X-31 is the first international experimental aircraft development program
administered by a U.S. government agency. It is one, if not the most,
successful effort initiated by the NATO Cooperative Research and Development
Program.

Click on Picture to enlarge

The X-31's first fully automated, thrust vectored landing was completed on
April 22, 2003.

Flight testing of the VECTOR X-31 ended April 29, 2003 at NAS Patuxtent
River, where it had first arrived in April 2000. This followed a week of
successful demonstrations of the world's first fully automated, thrust
vectored landings at up to 24 degrees angle of attack. For three years, the
VECTOR test team had been working to demonstrate the viability of thrust
vectoring for extremely short takeoff and landing, using the unique X-31 as a
test bed for the concept. During the last flight, the thrust-vectored jet
completed an automated ESTOL landing at 24 degrees angle of attack and 121
knots, a 31 percent reduction from the aircraft's normal landing speed of 175
knots.

The X-31 typically requires 8,000 feet to stop after a conventional
landing, but following the ESTOL touchdown, just 1,700 feet were needed to
slow the X-31 down enough to turn around in the middle of the runway and taxi
in a complete circle.

The program was then set to move into a data analysis and reporting stage,
creating would essentially amount to a how-to manual for thrust-vectored ESTOL
and the technology demonstrated on the X-31. , Young has been saying that the
product of the VECTOR program would be knowledge. "Our intention was to get
the data to aid government and industry in transitioning this technology to
production aircraft," she said.

The X-31 is the first international experimental aircraft development
program administered by a U.S. government agency. It is one, if not the most,
successful effort initiated by the NATO Cooperative Research and Development
Program.

The X-31 program logged an X-Plane record total of 524 flights in 52 months
with 14 pilots from NASA, U.S. Navy, U.S. Marine Corps, U.S. Air Force, German
Air Force, DASA, Rockwell International, and Deutsche Aerospace, flying the
aircraft.

Specifications

Designed and constructed as a demonstrator aircraft by Rockwell Aerospace,
North American Aircraft and Deutsche Aerospace.

The X-31 is a single seat aircraft with a wing span of 23.83 feet (7.3 m).

The fuselage length is 43.33 feet (1 2.8 m).

The X-31 is powered by a single General Electric P404-GE-400 turbofan
engine, producing 16,000 pounds (71,168 N) of thrust in afterburner.

Typical takeoff weight of the X-31 is 16,100 pounds (7,303 kg).

The X-31's normal flight envelope includes speeds up to Mach 0.9 with an
altitude capability of 40,000 feet (12,192 m). For specific tests to determine
thrust vector effectiveness at supersonic speeds the aircraft was flown to
Mach 1.28 at 35,000 feet.

The X-31 Configuration Evoluion

Click on Picture to enlarge

X-31 with F-16 canopy during tests in the Langley 14-
by 22-Foot Low-Speed Tunnel.

Free-flight tests of the X-31 in the Langley Full-Scale
Tunnel.

The Rockwell and MBB X-31 design team merged
their configuration candidates into a canard fighter powered by a single
General Electric F404 engine with a single vertical tail. The initial
design included an F-16 canopy for cost-saving purposes. Extensive tests
of the initial X-31 configuration were carried out at Langley during 1987.
These tests included static wind-tunnel tests and configuration component
evaluations in the Langley 14- by 22-Foot High-Speed Tunnel,
rotary-balance tests in the Langley 20-Foot Vertical Spin Tunnel to
determine aerodynamic characteristics during spins, and dynamic force
tests in the Langley Full-Scale Tunnel. Unfortunately, in 1988 the X-31
configuration was revised, and an F-18 canopy was incorporated. This
change was regarded as significant, and a major portion of the previous
wind-tunnel tests had to be repeated for the revised configuration.

Rotary-balance tests of the revised configuration were conducted in 1988,
and spin tests and static and dynamic tests were completed in 1989 for the
updated configuration. In 1989, a 0.19-scale model of the X-31 underwent
extensive aerodynamic and free-flight tests in the Langley Full-Scale
Tunnel. Results from these ground-based studies indicated that the X-31
might have marginal nose-down control at high angles of attack and that
the configuration might exhibit severe, unstable lateral oscillations
(wing rock) that would result in a violent, disorienting roll departure
and an unrecoverable inverted stall condition. Fortunately, the results
also indicated that a simple control law concept could prevent the
aircraft from entering a spin. The awareness that such phenomena might
exist for the full-scale aircraft enabled the X-31 design team to
configure the flight control system for maximum effectiveness.

An exhaustive test, which included 498 paddle and nozzle configurations
of the multiaxis thrust-vectoring system, was conducted by Langley
researcher Francis J. Capone in the Jet Exit Test Facility during 1988.
These data were used to select the final paddle and nozzle multi-axis
thrust-vectoring configuration. These data were also critical to the
design of the X-31 flight control system, since vectored thrust imposes
large forces and moments in addition to the normal aerodynamic parameters.

A 0.27-scale drop model was used by Langley to evaluate the post-stall
and out-of-control recovery characteristics of the configuration. The
model, which weighed about 540 lb and included extensive instrumentation,
was flown without an engine to assess the capabilities and characteristics
of the basic airframe. The objective was to demonstrate that the X-31
would be agile and have satisfactory characteristics without the
additional augmentation provided by thrust vectoring. The drop-model test
identifies characteristics and large amplitude flight motions that cannot
be assessed in conventional wind or spin tunnels. In the X-31 Program, the
technique proved to be invaluable as an early indicator of the highly
unconventional behavior of the configuration. In particular, the violent
roll departure indicated by tests of the free-flight model was encountered
during the drop-model tests. Several control schemes were evaluated to
eliminate this problem. In addition, the drop-model test technique
provided solutions to barrier problems during the full-scale flight-test
program.

Click on Picture to enlarge

One of the X-31 aircraft in flight.

Langley researcher Mark Croom (l) discusses the X-31
drop-model program with an X-31 program manager.

Mark Croom points to the aft-fuselage strakes defined
by
Langley tests and subsequently incorporated on the X-31 aircraft.

The first flight of the first X-31 aircraft
occurred at Palmdale, California, on October 11, 1990, and the second
aircraft made its first flight on January 19, 1991. During the initial
phase of flight-test operations at the Rockwell facility at Palmdale, the
two aircraft were flown on 108 test missions. On the test missions, the
aircraft achieved thrust vectoring in flight and expanded the post-stall
envelope to 40-deg angle of attack. Operations were then moved to Dryden
in February 1992, at the request of DARPA.

At Dryden, the International
Test Organization (ITO) expanded the flight envelope of the aircraft,
including military utility evaluations that compared the X-31 to similarly
equipped aircraft for maneuverability in simulated combat. The ITO,
managed by DARPA, included NASA, the U.S. Navy, the U.S. Air Force,
Rockwell Aerospace, the Federal Republic of Germany, and Deutsche
Aerospace (formerly Messerschmitt-Bolkow-Blohm). The first NASA flight
under the ITO took place in April 1992. As the X-31 full-scale aircraft
flight tests began at Dryden, the Langley staff maintained a close support
role for consultation and ground testing capability.

Two problems surfaced during the X-31 flight-test program, and both
were considered significant enough to curtail flight tests until solutions
were found. The first problem was encountered in the flight-test program
when it became apparent that the pitch control effectiveness of the
aircraft at post-stall conditions (particularly at aft center of gravity
conditions) was marginal. Pilots reported that their ability to obtain
positive, crisp, nose-down aircraft response was unsatisfactory and that
increased control effectiveness was required if the X-31 was to be
considered tactically responsive at high angles of attack. As part of the
X-31 Team, Langley was requested to conduct wind-tunnel tests to explore
options to provide the increased control at high angles of attack. Mark
Croom and his team quickly responded and evaluated 16 configuration
modifications to improve nose-down recovery capability in the Full-Scale
Tunnel. Results of the investigation recommended that a pair of 6- by
65-in. strakes be mounted along the fuselage after-body to promote
nose-down recovery. The Langley recommendations, which were given within a
week of the test request, provided a timely solution to the problem. The
aft-fuselage strakes were incorporated in the X-31, and the pilots
reported that the nose-down control was significantly improved.

The second problem that occurred in the X-31 full-scale flight test was
caused by large out of trim asymmetric yawing moments at high angles of
attack. Shortly into the high-angle-of-attack, elevated-g phase of
the envelope expansion, a departure from controlled flight occurred as the
pilot was performing a maneuver at 60-deg angle of attack. Data analysis
by the X-31 team indicated that a large asymmetric yawing moment, in
excess of the available control power, had triggered the departure. In
response to an urgent request for solutions, Croom and the Langley team
conducted tests in the Langley Full-Scale Tunnel to design nose strakes
that would minimize the problem. Once again, Langley responded rapidly
with a strake configuration that permitted the flights to continue.

The X-31 Program logged an X-plane record of 524 flights in 52 months
with 14 pilots from NASA, the U.S. Navy, the U.S. Marine Corps, the U.S.
Air Force, the German Air Force, Rockwell International, and Deutsche
Aerospace.

Evaluation of the X-31 as an enhanced fighter maneuverability
demonstrator by the ITO concluded in early 1995.

The Role Of The X-31 In High Angle Of
Attack Technology

Langley Flight Test Center NASA

Click on Picture to enlarge

The accompanying photograph shows three
thrust-vectoring aircraft, each capable of flying at extreme angles of
attack, cruising over the California desert in March 1994.

The F-18 HARV (top), the X-31 (middle), and the F-16
MATV (bottom) in flight.

Highlights of Research by Langley for the X-31

Langley contributed to exploratory studies of a
fighter configuration designed to exploit high angles of attack in a
precursor to the X-31 Program with Rockwell.

Working with Rockwell, Langley identified
unacceptable characteristics of the initial design and successfully
revised the configuration.

At the request of the Defense Advanced Research
Projects Agency, Langley participated in the International X-31 Program
and provided information on stability and control, control system effects,
configuration effects, thrust-vectoring system, spin recovery, and
recovery from out of control conditions.

During flight tests of the two X-31 research
aircraft at NASA Dryden Flight Research Center, Langley provided facility
support and technical consultation and analysis.

On two occasions, Langley provided timely
solutions to critical X-31 deficiencies that had stopped the flight
program.

The Rockwell (now Boeing) and Messerschmitt-Bolkow-Blohm (MBB) X-31
Enhanced Fighter Maneuverability (EFM) Demonstrator was designed to
demonstrate the effectiveness of controlled maneuvers at extreme angles of
attack during certain close-in air combat scenarios. The first International
(U.S. and Federal Republic of (West) Germany) X-Plane Program showed the value
of using thrust vectoring (redirecting engine exhaust flow) with advanced
flight control systems to provide unprecedented levels of controlled flight to
very high angles of attack. Whereas many previous fighters experienced loss of
control in this regime, the X-31 was able to maneuver without fear of loss of
control or inadvertent spins, which provided the pilot with new tactical
options. The X-31, along with the NASA F-18 High Alpha Research Vehicle, was
used in extensive flight tests at NASA Dryden Flight Research Center in the
1990’s to provide the technologies and tactical evaluations to remove the
high-angle-of-attack “barrier.”

Langley became involved in the X-31 Program in 1984 in a cooperative
research program with Rockwell to develop a fighter configuration capable of
highly agile flight at extreme angles of attack. Free-flight model tests at
Langley led to a major redesign of the Rockwell candidate configuration. When
Rockwell, the Defense Advanced Research Projects Agency (DARPA), and the West
Germans formed the X-31 Program, the staff at Langley was requested to
participate in the configuration development. Langley researchers conducted
extensive studies of the stability, control, and thrust-vectoring system of
the vehicle. Langley remained active in the program as Dryden became the
responsible test organization during the flight tests of two X-31 demonstrator
aircraft. Flight tests began at Dryden in February 1992 and concluded in 1995.

During the flight evaluation tests at Dryden, Langley provided technical
support and on two occasions provided rapid solutions to critical stability
and control problems that had stopped the flight tests.

Langley support of the X-31 included tests in the 30- by 60-Foot
(Full-Scale) Tunnel, the 20-Foot Vertical Spin Tunnel, the 12-Foot Low-Speed
Tunnel, the 14- by 22-Foot Tunnel, the 16-Foot Transonic Tunnel, the Jet Exit
Test Facility, a radio-controlled drop model, and piloted simulators.

The Langley Contribution To The X-31

Langley Flight Test Center NASA

Background

Langley participated in the X-31 Enhanced Fighter
Maneuverability (EFM) Program during four separate activities. From 1973 to
1984, Langley was active in the planning, testing, and analysis of the
remotely piloted Highly Maneuverable Aircraft Technology (HiMAT) research
vehicle. From 1984 to 1985, Langley cooperated in a program with Rockwell
International to develop a representative fighter configuration that could
demonstrate the advantages of exploiting high-angle-of-attack maneuvers during
close-in air combat. From 1986 to 1991, Langley participated in the analysis
and configuration development of the International (United States and Federal
Republic of (West) Germany) X-31 Program. From 1991 to 1995, Langley supported
the flight-test program, which was conducted at NASA Dryden Flight Research
Center by the International Test Organization.

The HiMAT Program

Following the Vietnam conflict and renewed emphasis on close-in
air-to-air combat, the U.S. military became interested in aircraft
maneuverability. As a result, the requirement for high speeds, long considered
the key factor in successful air combat, became a secondary objective. NASA
initiated a joint program with the Air Force known as the Highly Maneuverable
Aircraft Technology (HiMAT) Program. The staffs of the Langley, Ames, and
Dryden Research Centers all participated in planning the HiMAT Program, with
William P. Henderson serving as the technical lead and coordinator for
Langley. The focus of the HiMAT Program was flight research and
maneuverability demonstrations of a representative advanced configuration in
the form of a remotely piloted subscale vehicle at Dryden. The goals of HiMAT
included a 100-percent increase in aerodynamic efficiency over 1973
technology, and maneuverability that would allow a sustained 8-g turn
at a Mach number of 0.9 and an altitude of 25,000 ft. The program ultimately
achieved all goals.

The X-31 Aircraft
At The Paris Air Show

Click on Picture to enlarge

In another aviation first, the
unique maneuvering capabilities of the X-31 high-performance experimental
fighter aircraft were demonstrated Saturday to the international aerospace
community in a performance at the 1995 Paris Air Show. The X-31's
performance is the first international air show flight demonstration by an
X-plane.

The X-31's demonstration included a series of unique maneuvers in
which the aircraft dramatically exceeded the aerodynamic stall angle, a
condition in which ordinary aircraft lose control. The X-31 is able to
exploit this high angle-of-attack "post stall" capability to turn and
maneuver more quickly and over shorter distances than can conventional
aircraft.

The specific maneuver set demonstrated during the air show included a
post-stall loop after takeoff, followed immediately by a rapid, so-called
"helicopter turn" in the opposite direction; a low-altitude, horizontal,
post-stall break turn termed the "mongoose"; a slow-speed, high
angle-of-attack turn in the opposite direction called the "Herbst turn";
and, finally, a climbing, high-speed entry into a post-stall loop, followed
by rapid, sequential re-pointing of the aircraft in opposite directions.

In preparation for the low-altitude air show demonstration, the X-31
had conducted 34 flights in less than a month. This represents a record for
X-aircraft, bettering the previous achievement of 22 flights during one
month; the previous record was also held by the X-31.

Two X-31 experimental aircraft were built and flew during a four-year
exploration and test program to demonstrate the feasibility of thrust
vectoring control in the post-stall flight regime. The X-31 used maneuvers
similar to those in its air show repertoire in mock, close-in air combat
engagements against a variety of front-line fighter aircraft, dramatically
dominating many of these "adversaries."

The X-31's maneuvering achievements have also been complemented by
another significant aviation first when the aircraft demonstrated that
flight without a tail is possible at supersonic as well as subsonic speeds.
Designing aircraft without tails offers the potential for reduced weight and
increased performance, efficiency and stealth. The X-31 demonstrated flight
without a tail through a novel supersonic in-flight experiment in which the
flight control system was fooled into reacting as though the aircraft had no
tail. The thrust vectoring capability was then used to provide necessary
aircraft stability, trim and control.

Click on Picture to enlarge

One aircraft crashed during a test flight in January 1995, after a
departure from controlled flight not attributed to any of the aircraft's
unique systems or maneuvering capabilities. The remaining X-31 aircraft was
brought back to flight status in April.

The X-31 aircraft was developed jointly by Rockwell International's
North American Aircraft Division and Daimler-Benz Aerospace (formerly
Messerschmitt-Bolkow-Blohm), under sponsorship by the U.S. Department of
Defense and the German Federal Ministry of Defense. The program has been
operating under the auspices of the X-31 International Test Organization
(ITO) from the NASA-Dryden Flight Research Center, Edwards, Calif. The ITO
is comprised of participants from the DoD's Advanced Research Projects
Agency, NASA, the U.S. Navy, U.S. Air Force, the German Government, German
Air Force, and the two prime contractors, Rockwell International and
Daimler-Benz. Note: Video footage of the X-31 performing maneuvers similar
to those performed at the air show is available from Ken Carter, Room 2E765,
Pentagon, at (703) 697-6161.